There are two important design features that have been consistently used in the heater head construction for Stirling engines for automotive use. First, the heater head employs a plurality of small heat transfer tubes (heater tubes), which communicate with an associated regenerator-cooling apparatus to complete a closed working fluid curcuit. Typically these heater tubes have been constructed of high temperature metals, such as austenitic stainless steel, or nickel based or cobalt based heat resistant alloys. Secondly, the working fluid is selected as hydrogen and maintained under an operating pressure which is extremely high, i.e. in the range of 50-200 atmospheres. The necessity for the use of hydrogen under extreme pressure is to achieve rates of heat and mass-transfer which will make the thermal efficiency and specific output of the engine tolerable and within design goals.
However, the use of these two features wherein high pressure fluid is closed within a maze of small diameter tubing for heat absorption, has created several problems. One of the most important and frequent problems is that of distortion and cracking of the metallic tubes under severe thermal and mechanical stresses created during operation of the engine. The mechanical bursting stresses are obvious in that the high pressure within the internal volume of the small diameter tube is considerably greater than the pressure surrounding the outer wall of such tube which is typically at ambient pressure conditions. In addition, thermal stresses are generated by the extreme thermal gradient across the tube walls, the temperature of the gas surrounding the tubes being over 2000.degree. F. and the temperature of the closed working gas being in the range of 1200.degree.-1800.degree. F.
To insure that the maze of heater tubes are not destroyed by such mechanical and thermal stresses, the art has turned to exotic materials. But even with the use of exotic heat resistant alloys, the stresses have increased the probability that such tubing will have a limited life potential.